Darksiders has a glitch in the Ashlands area. Outside of one of the pits is a trio of horse-riding demons. If you jump into the pit and crawl back out of it, a trio of riders will respawn regardless of whether or not you killed the former ones. You can do this enough times to completely fill the area with riders to the point of being able to crash the game. Some gamers like to exploit this bug in order to quickly gain souls and sword experience.
Batman: Arkham Asylum and Batman: Arkham City have an absolutely hilarious glitch with the ground takedowns. Rather than crouch on the poor mook's chest and hit them in the head, occasionally, Batman will crouch on their head and give them a ball-busting punch in the nuts. An even rarer version has them flipped over as well, leading to Batman punching a man in the ass so hard he's knocked out. And an extremely rare one has him punch someone as a thug is getting up off of the ground, leading Batman to punch them up the ass hard enough they get sent flying. Catwoman sometimes has similar jank, where she appears to knock thugs out just by slapping their ass.
The Multi-Ground Takedown can sometimes knock thugs across the street or ridiculously high into the air.
In Arkham City, perching on a billboard or on the fences that some building roofs have and facing the corner, and then rapidly shooting quickfire freeze grenades will have Batman slowly inch towards the edge, at which point, he'll start rising into the air. Go high enough, and you can escape the game boundaries and fly over the walls to explore the surprisingly detailed Gotham City (though there is no collision detection so you only get one glide), or out across the waters at Arkham Island, fully rendered (again with no collision detection). Falling through the void-out plane will have Batman use his Grapnel Gun, which has apparently been upgraded to be able to shoot across a lake, to get back into the normal playing area.
Batman: Arkham Origins has a hilarious semi-rare bug that occurs if you leave an enemy-planted mine alone along the path a cutscene happens towards, where Batman and Harleen Quinzel walk along and, well...
The PS version had a really weird one: A bug in the game would let you warp directly from Ron's house to the Gryffindor Common Room way later in the game. However, you warped directly there without passing GO (or collecting 200 Every Flavour Beans, but anyways) which meant that other events in the game that should have been seen by now weren't, and when you go to where those events took place, you'd see those events - except out of order. One particular event that can be skipped "merges" with another event later, and the characters will spout random lines from both events. The game won't move on if you just let it sit in an endless loop like this so you have to keep pressing the square button and hope that you get out of glitched speech hell. Wow.
Back to the PC version, there's a point in Lockhart's spongify challenge where you have to fight three imps (which are like gnomes but faster and much more difficult) to move on. You can instead stay very close to the wall and prevent the platform from lifting and the imps from coming out in the first place, thus saving yourself time and aggravation.
Also in the PC version, it's possible to get an unlimited number of Every Flavor Beans in the greenhouse part when you collect potion ingredients. All you need to do is stand in front of a row of two flower pots, and cast at the one behind the nearest to you. Continue casting, and beans will come out of it even after it's been knocked over.
In the Crusader games: drop an ammo-using gun. Pick it up. Its clip is now full. Ta-daaa!
The first five games have corner bugs, wherein jumping into a corner at the right angle makes the game think you are on top of it, causing you to warp to the top. This allows access to normally inaccessible areas, and therefore massively increases the amount of possible exploration in the games (as well as being helpful for things like Speedruns).
The fence bug allows you to slip through gaps between fences. One notable use of this is at the start of the Obelisk Of Khamoon stage. If Lara is positioned just right, she can bug through the fence, allowing her to regain access to the previous level. See here.
The trigger bug. Enemies appear when Lara steps on certain floor tiles. In some cases it is entirely possible to glitch past these tiles, preventing enemies from spawning.
In Tomb Raider The Angelof Darkness, performing the level skip cheat and selecting the first level allowed Lara to climb down the ladder at the end of the stage without obtaining the gate key beforehand. By doing this, the cutscene to end the level is never triggered, allowing the player to explore a hidden training level which was removed from the official release. Despite being incomplete, it is interesting to note that the tutorial explains how to perform certain actions essential to the gameplay.
The ZX Spectrum game The Great Escape. You can give chocolate to a prisoner, who'll distract the guards in exchange for the bribe gift ... but you can also offer it to the stove, which will start moving around your room (to the same in-game effect). Also, unlocking a door improves your morale (which serves as the game's Life Meter); but nothing prevents you from repeatedly using the key on the door after it has been unlocked.
Little Big Adventure 2 has the famous "jump-save-bug", which allows players to jump to infinite heights by saving and loading the game in mid-jump. This allowed skipping large parts of the game and reaching places which are normally unreachable.
Among the many uses for this glitch are obtaining the jetpack at the start of the game (normally obtained after returning from Zeelich, way later in the game), and skipping said return from Zeelich by simply jump-save-bugging out of the prison area to the rest of the planet.
You can also grab the Pearl of Incandescence early (with the bug itself or with the jetpack you got with it) and thus the Ring of Lightning. If you haven't even picked up your Tunic yet, you won't have a magic gauge yet, but the Ring of Lightning only works with a full bar of magic. So... the game gets confused, and lets you spam the move endlessly (by using the hotkey)! Unfortunately, it's limited to level 1 magic; as soon as you upgrade your magic to Green or Red, you'll get a magic bar so this spamming glitch goes away.
The Jump Save Bug is also useful for triggering the second visit to Zeelich (which is around halfway into the game, normally) minutes into a new game if used alongside the Ferry Bug to get to desert island early. Simply go to the temple of Bu, find the place where the employee is working on some fenced off area, and jump over it to get to the hidden base with the spaceships. In this way you can skip half the game, and beat the game without even picking up your Tunic and (maybe) Magic Ball!
There was a glitch in Onimusha: Warlords in which you could juggle an airborne enemy. This glitch was fixed before the game was released, but the testers thought it was so cool that they had to use it somewhere. Thus the combat system for Devil May Cry was born.
In Lego Star Wars on the Wii, there is a wonderful bug where any bonus activated will remain activated if you quit and load a different save slot.
The DS version of LEGO Star Wars: The Complete Saga lets you fly as Jar Jar(Who you otherwise have no reason to play as) or a custom character with a blaster by double-jumping with your blaster out and shooting in midair. You can then moon jump without hacks by mashing B and Y over and over. It can also be done as Jango and Boba Fett, but it's not as easy.
The console versions of the game have a glitch dubbed "the triple jump", where by double jumping as any lightsaber wielding character, preforming the slam attack, but changing characters right as you press the attack button will give you an extra boost. This is best done as Grievous in Prequel Trilogy levels and the Emperor in Original Trilogy levels. Unfortunately, this glitch was removed in newer games.
LEGO Jurassic World has the "Haunted Gyrosphere" appear in the level "Gyrosphere Valley". Zach is supposed to be trapped inside of it, but if you can get free by intentionally driving into water, switching to Gray just as it explodes, and back to Zach as he respawns. The Gyrosphere will respawn beside him, and follow you around as if it were a third teammate. It also happens to be quite useful since now you're free to hop in and out of it, making the stage puzzles much easier to solve than normal.
Yume Nikki has a useful glitch that can boost your speed to the max RPG Maker 2000 speed (8x, if anyone's counting), making you amazingly fast. You just had to sit at your desk in the dream world, use an effect (using the bike effect made you even faster) and get off the chair. Especially useful for finding the rest of the effects!
Arguably an abstract Game Breaker, as Yume Nikki is designed to be a psychological sight-seeing game and isn't meant to be experienced at warp speed. Using the glitch, the player can easily and unwittingly breeze past the majority of the Easter Eggs and significant scenery in the game, making the experience something of a moot point. Definitely useful for those playing multiple times, though.
In Adventure, having too many items on screen would break the collision detection. This meant that sometimes your sword would not kill a dragon. It also meant you could phase through a dragon's body without getting hurt. Given that you would often be unarmed due to the one-item limit, and that you don't actually have to kill any of the dragons to beat the game, the good outweighed the bad. This bug is even mentioned in the manual. This bug was even exploited by the game's creator; without the bug, the game's Easter Egg wouldn't be possible.
The ancient Apple game Aztec (a side-scrolling Indiana Jones ripoff) had an AI that was less The Computer Is a Cheating Bastard and more The Computer Is a Sociopathic Monster. Fortunately, it was buggier than a Florida swamp. One of the best ways to escape a trap was to draw your machete and lunge forward over the boundary between rooms. Once you got to the halfway point, you'd fall to the next level and escape the trap. A related bug allowed the player to climb through the ceiling, walk up his own score and onto the floor of the room above. The crowner, though, was using a stick of dynamite at the beginning outside the tomb: it would bug the game into thinking you were on the 8th level, right near the idol, although with the caveat that you were now in the most hostile part of the tomb with no weapons or ammo.
In King's Quest: Mask of Eternity, scaling a wall with the rope and hook in the Underground Realm of the Gnomes could result in Connor ending up on the other side of the walls bordering the level. Weirdly, it wasn't just an empty wasteland - there were rocks lying around that could be picked up and zombies to fight.
In Beyond Good & Evil, the ramp to the left of the item stand is particularly easy to clip through. While clipping through it usually leads to falling into the abyss, doing a forward roll and aiming properly lets you hit the warp space for a nearby door and get to one area earlier than you're supposed to. Using this to pick up an extra Plot Coupon earlier than you're supposed to allows you to buy an item that will get you past a Broken Bridge one dungeon too early. This causes tons of random dialogue glitches, total confusion on the part of your mission log, and, ultimately, a game freeze if you try to go to the next dungeon. Watching the game freak out is worth it, though.
In PAL version of ICO it's possible to use a glitch that allows Ico to jump a little higher than normal, and in some cases do really huge leaps. This allows for skipping large parts of game, including an entire wing of the castle. It works only in 50 Hz mode, though.
Legacy of the Wizard has the pause-jump bug, where pausing and unpausing the game will allow any character to jump again from mid-air. This makes it possible for some characters to reach areas they're not supposed to.
In the original version of La-Mulana the player could jump through the wall on the first floor of the Infinite Corridor and end up on the bottom floor, allowing the player to bypass an entire floor full of annoying block puzzles.
In Beyond Castle Wolfenstein it was possible, through judicious exiting and re-entering rooms, to actually "erase" Nazi guards from rooms. Given that wasting your time showing your pass everywhere slows you down when you need to leave the castle, this can be a time-saver.
In Star Fox Adventures, it is ra— er, unlikely but possible, for the bat in one of the structures in DarkIce Mines to respawn in a wall and get stuck forever, or at least until you kill it or leave and come back. The respawn animation will loop forever, as well.
At Dragon Rock, pause the game while looking at the Earthwalker. His tail will disappear. After you free him, you can even look at him from the back with his tail gone.
Star Wars: The Force Unleashed had a brilliant bug where you could max out the level meter and gain permanent invulnerability and infinite force energy.
Every cheat that gives you anything worthwhile also disables your ability to save, effectively making cheats useless from a gameplay point of view. However, activating a cheat, dying, and quitting and reloading just at the right moment will remove the cheat's adverse effects while letting you keep the bonuses.
Because of the way Boktai 3 writes player-character's data to memory, it's possible to obtain Sabata's dark gun and Zero Shift spell through a carefully timed reset. The gun itself is useless (the game crashes), but the dark lens is quite useful on some enemies and Zero Shift becomes a free and superior version of the Dash once you meet Cheyenne.
In One PieceUnlimited Cruise 2 if you dash, swap movesets, and dash again in rapid succession your character returns to his/her standing pose and keeps sliding forward (steering is possible, if slippery). This can be useful for quick travel, since you move as fast as if dashing, without consuming SP. Mostly useful on the first island, since the latter ones require some jumping.